Oilpan: fix Windows build after r178431.

With http://crrev.com/406523002 making GarbageCollectedMixin a non-abstract
class, MSVC elicits a warning that DLL exported GCMixin classes have a
base class that isn't DLL exported.

Add the needed annotation to GarbageCollectedMixin to address (as warnings
are considered as errors.)

R=haraken@chromium.org,wibling@chromium.org
BUG=382557
NOTRY=true

Review URL: https://codereview.chromium.org/396153003

git-svn-id: svn://svn.chromium.org/blink/trunk@178439 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/heap/Visitor.h

@@ -584,7 +584,7 @@ template<typename T> bool ObjectAliveTrait<T>::isAlive(Visitor* visitor, T* obj)
 // Note that this is only enabled for Member<B>. For Member<A> which we can
 // compute the object header addr statically, this dynamic dispatch is not used.
 
-class GarbageCollectedMixin {
+class PLATFORM_EXPORT GarbageCollectedMixin {
 public:
     virtual void adjustAndMark(Visitor*) const { };
     virtual bool isAlive(Visitor*) const { return true; };